A great variety of level gauge systems are known in the art, and for many different applications. For some applications it is requested that an alarm is issued when the filling level exceeds a certain limit. For reasons of security, in a container, in almost all cases, a cargo which requires a closed gauging device also requires at least one of a high level alarm and an overfill control. Such safety systems are e.g. often mandatory on e.g. tankers. According to safety rules the high level and/or overfill alarm system must normally function independently of an ordinary level gauge system for measuring at all levels. A fault of one system must not render the other system(s) inoperative.
The systems producing alarms are conventionally realized as mechanical systems, including a float member which is mechanically connected to a detector. An example of such a system is e.g. disclosed in U.S. Pat. No. 5,649,450. However, such mechanical systems are the subject of many problems. For example, the environment in the tanks is often relatively rough, making the movable mechanical parts likely to malfunction over time. Further, the floating member needs to be in contact with the fluid, which is disadvantageous since the surfaces normally not calm in use. Accordingly, these mechanical system have problems with robustness, accuracy and reliability.
Still another disadvantage of a fixedly mounted float detector is its inability to adjust the alarming level for tank inclinations, should the device not be located in the tank center.
Further it is known to use non-contacting level gauge systems, such as radar level gauges. These devices utilize antennas to transmit electromagnetic waves toward the material being monitored and to receive electromagnetic echoes which are reflected at the surface of the material being monitored. Such systems could either use continuous transmitted signals, so-called FMWC (frequency modulated continuous wave) or pulsed transmitted signals. Examples of such radar level gauge systems are e.g. disclosed in WO 2004/018978 and U.S. Pat. No. 6,414,625.
A common problem in alarm level gauge systems is that in the design stage of a tank, the maximum volume allowed in the tank is conventionally estimated and decided by a third party beforehand. This maximum volume is not to be exceeded when the tank is set in operation. Also, the operator/class normally requires a warning on a volume close to the maximum allowed volume to simplify the operation when loading the tank, which is usually referred to as “high level”. Accordingly, the alarm level gauge system is designed according to this estimated and predetermined information, and arranged in the tank. Thereafter, during commissioning, the tank is inspected and often enough the inspector allows the tank to hold a greater volume than initially stated during the design stage, which puts the tank owner in a dilemma; either the level gauge system must, which is typically mechanically fixed, be replaced or modified, which is normally a tedious, cumbersome and expensive operation, or the less than optimal tank volume has to be accepted, which means that the full storage capacity could not be used. Both options cost a lot of money for the customer in terms of inefficiency in use and/or delayed projects and longer time in port for the tankers.
There is therefore a need for a more effective alarm level gauge system.